Basic Guidelines for Tuning with the XPS Motion Controller
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TECHNICAL APPLICATION NOTE Basic Guidelines for Tuning With The XPS Motion Controller 1.0 Concept of the DC Servo frictionless surface at a rest position and a perturbation is applied by pulling The XPS positions the stage by optimizing error response, accuracy, and stability the spring and if the Kp (Hooke’s) value were zero, then the mass would not by scaling measured position error by the correctors Proportional, Integral, and move to correct for the perturbation. With an appreciable Kp>0, the spring Derivative (Kp, Ki, Kd). The XPS responds to measured position error (Setpoint - responds and is put into oscillation. Encoder) is indicated in Section 14.1.2.1 of the XPS user manual: Similarly, if the DC Servo were a perfectly frictionless system, then Kp would send the system immediately into oscillation as it corrects for position. Like a spring, the value of Kp results in the speed of response to the error: like a stiffer spring, a higher Kp will cause the stage to return more quickly. 1.2 Damped Harmonic Oscillator A real stage system, however, always has some friction or an electronic dampening term. The spring mass system shows that the magnitude of the error The Kform term in the left diagram is set to zero for standard stages. The Kform (periodic overshoot) is diminished by Kd. For a DC Servo, the Kp and Kd terms are with variable gains is a tool for changing the PID parameters during the motion sufficient to cause the stage to respond to an error and to settle the oscillations. and reserved for larger assemblies and special stages. Standard DC Servo stages can have position certainty and stability set by the simplified diagram on the right. The diagram on the right can be written in the form below which provides an analogy for understanding the relationship between the corrector parameters Kp, Ki, and Kd. de(t) Output = Kp e(t) + Kd + Ki e(t)dt 1.3 Following Error dt In the spring system above the rest position and the target position are the same, The relationship between Output and the terms Kp and Kd can be considered as a X=0. Consider a situation where the rest position after responding to the error damped harmonic oscillator like a spring mass system or an LRC circuit. The Ki (Kp) and dampening the error (Kd) is displaced from the target position. This term is an integral of the error over time, hence it applies gain to the collection of scenario can be illustrated by putting the spring mass system into the gravity field error over time: the Ki term is the gain for steady state error. Similarly, it can be where gravity G applies a force away from the target position. seen that the Kd term multiplied by the derivative in time applies gain for fast changes like a damper would. Finally, Kp applies gain for the instantaneous error in time and can be seen to speed the adjustment of error in the system by appling an immediate response. In the detailed description of the simple harmonic oscillator, the behavior of each corrector (Kp, Ki, Kd) becomes apparent. 1.1 Simple Harmonic Oscillator In a simple spring mass system analogy, Hooke's constant is to the stretch in the spring as Kp is to error in position of the DC Servo. If the mass is set on a TECHNICAL APPLICATION NOTE Basic Guidelines for Tuning With The XPS Motion Controller In a speed control loop, Kd parameter is redundant and normally avoided, but at higher values from Kp & Ki, Kd can help improve the “tightness” of the transient response. The lack of derivative action in a speed control loop may make the system steadier in the steady state as the derivative action is more sensitive to higher frequency terms in the input. Kp, proportional gain, can drive the cut-off frequency of the closed loop. And integral gain, Ki, has the capability to overcome perturbations of physical or mechanical imperfections and to limit static error. 3.0 Summary of Correctors Kp, Ki, and Kd In the image above, before any stretch is applied to the spring, the rest position is The analysis of the Servo system starting from the corrector diagram in the set to G instead of the target of X=0. Hence, after the spring is stretched and Kp XPS user manual section 14.1.2.1 are summarized below: responds and Kd dampens, the final position will have an error. This error is seen as position Error = -0.3mm in the image below: this error is the following error Parameter Function Value Set Too Low Value Set Too High where Kp and Kd have no affect on it. Kp Determines stiffness of Servo loop too soft with Servo loop too tight with servo loop high following errors oscillations No matter how quickly (Kp) or how well damped (Kd) the following error exists. Ki Reduces following errors Stage does not reach or Oscillations at lower during long moves and at stay at the desired position frequency and higher Following error requires a closer look into the harmonic oscillator analogy since stop amplitude Kp and Kd to not affect following error. Kd Dampening factor used to Oscillations caused by other Oscillations at higher reduce oscillations parameters being too high frequency and audible noise In the spring mass system, the displacement from motor caused by large error G can be corrected by a pneumatic ripple in motor voltage cylinder or some other force that brings the The table below shows the result of increasing each parameter on a DC servo: position back to X=0 from the following Parameter's error position X=G. The correction of Effects Kp Ki Kd following error requires a second step: the Overshoot Kp Increases O.S. Ki Increases O.S. Kd Reduces O.S. system response and oscillations are first Rise Time Kp Reduces R.T. Ki Increases R.T. Kd Increase R.T. optimized then the following error is corrected with Ki. The corrector force Ki can impart an oscillation to the system Following Increasing Kp Reduces F.E. Ki Reduces F.E. robustly Kd has NO EFFECT on F.E. when its value is too high. Hence, the Ki Error when Kp is very small term is applied cautiously and to a system that is well behaved. 3.1 Relationship between Kp, Ki and Kd - PIDFFVelocity 2.0 Speed Control Loop In the following example, a demonstration of how the relationship between Kp, Ki From the above example of the spring mass system, the analogy is a good and Kd by correcting a DC Servo waveform using the table above. This example is representation for stages that are controlled by force/torque, also known as gathered with a velocity corrector loop stage (PIDFFVelocity), VP-25XA. PIDFFAcceleration corrector. In following flowchart, a system can be controlled by speed where the PID parameters objective is to make the actual motor speed match the ideal motor speed. In the below image provides a representation of a PID loop with a speed input. TECHNICAL APPLICATION NOTE Basic Guidelines for Tuning With The XPS Motion Controller The waveform on the above image is the initial state where the value of Kp is high 3.2 Relationship between Kp, Ki and Kd - PIDFFAcceleration and Ki and Kd is low or set to zero. This is when the DC Servo is oscillating and may In the next example, a demonstration of how the relationship between Kp, Ki also have an error off of its target position. But notice with just Kp, the output and Kd by correcting a DC Servo that is based off from an acceleration response is increased and at the steady state has a low following error. corrector loop (PIDFFAcceleration) using an XMS100 stage. In an acceleration corrector loop, also known as a force/torque loop, the derivate term drives the cut-off frequency of the closed loop and must be adjusted first. The waveform on the below image is the initial state where the value of Kd is high and Ki and Kd is low or set to zero. This is when the DC Servo is oscillating and may also have an error off of its target position. Notice that the plot shows a high frequency. The first approach is to reduce oscillations by reducing or eliminate Kp. Notice that the resulting following error at the rest position is further from zero. Now that the oscillations have been reduced, the steady state error is apparent and Ki may be increased to reduce the steady state error, resulting in the image below. Lowering the value of Kd will reduce the high frequency and oscillation shown in the plot below. Kd will improve the transient response, but the steady state error is apparent and needs to be improved by increasing Kp. The following error plot above has no apparent steady state error but is oscillating during the transient response. Introducing Kd will reduces the high frequency oscillations resulting in the low frequency error shown in the below image. Notice as Kd increases to dampen and stabilize the transient response, but the settling time has increased a bit. For stages with a velocity corrector loop (PIDFFVelocity), Kd is not always needed and the above plot is suitable for most applications. A high Kd can add high frequency errors. TECHNICAL APPLICATION NOTE Basic Guidelines for Tuning With The XPS Motion Controller The following error plot above has a lower apparent steady state error and 5.2 Select Stage to Tune still oscillating during the transient response.